A number of motor-operated valve test equipment patents have been issued in recent years, reflecting an industry interest in this area. The interest recently increased for testing at nuclear power plants because of the U.S. Nuclear Regulatory Commission's Generic Letter 89-10, Safety-Related Motor-Operated Valve Testing and Surveillance, issued Jun. 28, 1989, with three subsequent supplements. The generic letter relates to the Commission's concern for motor-operated valve operability and requests nuclear power plant license holders to increase their testing commitment to motor-operated valves.
In NUREG/CR-5558, "Generic Issue 87: Flexible Wedge Gate Valve Test Program," January 1991, the Commission reports much higher valve stem thrust requirements than previously expected necessary to operate gate valves under high fluid system differential pressure conditions. In the same report, pages 54-59, what is now called the "rate of loading" effect, is also discussed. The effect is related to use of a motor-operator torque switch commonly used in the United States and elsewhere in an operator manufactured by the Limitorque Corporation. The switch is used for stopping the operator motor on reaching a setpoint that limits the mechanical output of the operator.
In describing this rate of loading effect, the Commission reports, on page 54, that the stem force at torque switch trip is greater when the valve stem is loaded at a faster rate and further indicates that this effect is not yet fully understood. Unfortunately, commonly used test systems are used to adjust the motor-operator torque switch under high rates of loading; but, under required use of the valve to perform a fluid control function, stem loads are changed more slowly as the stem moves in the desired direction. Hence, operator testing results in a higher expectation of thrust output than actually available for use in operating a valve when it is needed.
This rate of loading effect, coincident with the higher than expected valve stem thrust requirement for fluid flow control noted in the same report, represents a divergence between requirements and capability. The condition has actually been recognized in the industry for over two years and needs to be addressed, particularly for valves that perform an important safety function as commonly found in a nuclear power plant.
Existing diagnostic test systems are used on rising-stem, motor-operated valves to set the torque switch to trip and de-energize the operator motor at a specific value of stem thrust. The thrust is measured either by direct or indirect means involving measurements taken when the stem is loaded very rapidly:
1) as the valve closure element is driven tightly into the valve seat, a characteristic use of R. L. Leon, U.S. Pat. No. 4,805,451,
2) on driving the valve stem downward into a load cell which blocks further travel, as with O. G. Crass, U.S. Pat. No. 4,570,903, or
3) on driving the valve stem upward into a load cell as with J. A. McNennamy, et. al., U.S. Pat. No. 4,693,113.
It is a characteristic associated with using all three of the test systems represented by these patents that the upper limit of the load is controlled by control means within the valve operator, the rate of loading is not controlled, and the rate is very high relative to normal loads developed when using the valve to control fluid flow.
It would normally be desired to increase the torque switch setpoint to compensate for this effect when using test systems subject to the rate of loading problem; but, the effect has not appeared to be readily quantifiable to allow adjustment. Additionally, existing systems that use valve seating or a load cell to develop thrust can result in the final thrust going significantly past the torque switch trip point. This is a result of the mechanically rigid nature of the equipment under test conditions, the time it takes for the operator motor controller to de-energize the motor after the desired stem load is achieved, and the motor inertia. Hence, the upward adjustment of the torque switch is limited by the same test technique that causes the need for it.
There currently is no commercial system available for testing and adjusting the torque switch that avoids the rate of loading problem. This includes the patented systems already noted.
Another solution might involve avoiding the problem by adopting a motor-operator control scheme that is not dependent on use of the typical torque switch. This could be through bypassing the switch in the region of stem travel where high stem thrusts are expected or eliminating it altogether as a control element. However, these same test systems, as noted above, generally depend on an operator torque switch to limit the induced stem load during testing. Hence, the torque switch and available test equipment exist in a symbiotic relationship where operator testing is prescribed, making the choice of eliminating the torque switch normally unacceptable.
In addition to the rate of loading problem, further review of the current art indicates a need to:
1) more realistically simulate the motor-operator function of driving the valve stem against a mechanical load, PA1 2) allow collecting data under quasi static load conditions to better determine operator mechanical condition and efficiency, detect problems developing, and obtain test data adequate to allow an extrapolation of results to an extended range of operating conditions, PA1 3) provide meaningful data for later comparison and trending in a periodic test or surveillance program.
An obvious solution for applying a controlled load for the purpose of testing might include a hydraulic cylinder and piston, and such devices have been and still are used successfully, but in special test applications and studies. Other devices might be used as well, including a spring having the desired travel and spring rate or a pneumatic cylinder and piston. There has been no known successful attempt, however, to adapt such loading means to a test methodology that can be used routinely in the field to resolve the rate of loading problem or for the advantages noted above.